SIGN STRUCTURES GUIDE SUPPORT DESIGN FOR UK TRAFFIC SIGNS

Transcription

1 SIGN STRUCTURES GUIDE SUPPORT DESIGN FOR UK TRAFFIC SIGNS Fully revised to BS EN :2007 May 2008

2 Published by the Institute of Highway Incorporated Engineers De Morgan House 58 Russell Square London WC1B 4HS Tel: Fax: Design Wendy Hooper: WDH Publishing Services Ltd, Essex Simon Morgan First published September 2007 Revised edition published May 2008 All rights reserved. No part of this publication shall be reproduced without the written permission of the publishers Institute of Highway Incorporated Engineers ISBN:

3 Sign Structures Guide Support design for UK traffic signs to BS EN :2007 May 2008

4 ACKNOWLEDGEMENTS The Institute of Highway Incorporated Engineers would like to thank the following for producing this guide: Editor Simon Morgan, Buchanan Computing Authors Jim Gallagher, Highways Agency Kirsten Morris, Mott MacDonald Simon Morgan Design Wendy Hooper, WDH Publishing Services Ltd Simon Morgan The example calculations were prepared by Mott MacDonald for work commissioned by the Highways Agency. About the authors All the authors have been closely involved with the preparation of guidance for the new UK National Annex to BS EN Simon Morgan, MA, MSc, CEng, CITP, MICE, is managing director of Buchanan Computing and a director of Colin Buchanan. He is a member of a number of national bodies related to traffic signs, including the BSI committee responsible for BS EN and its National Annex, and is the author of SignPlot and SignLoad software for the design of sign faces and sign structures. James Gallagher, BSc(Hons), CEng, MICE is senior structures advisor at the Highways Agency in Manchester. His responsibilities include the introduction and implementation of both BS EN and the passive safety standard BS EN on the English trunk road network. Kirsten Morris, BSc, CEng, MICE, FIStructE is a structures design team leader at Mott MacDonald in Southampton. She manages the structural design of sign supports and foundations for the Highways Agency Area 3 and has led the modification of the design method for signs to comply with the Eurocodes. She has experience in providing technical advice for the development and use of passively safe posts as sign supports. 2 SIGN STRUCTURES GUIDE

5 CONTENTS Acknowledgements...2 Contents Introduction Background Information on Wind Load Background Information on Support Design Foundation Design...12 Appendix A: Table NA.2 of National Annex (omitted from this version)...13 Appendix B: Flowchart for Determining Wind Load...16 Appendix C: Examples...17 Appendix D: References...31 SIGN STRUCTURES GUIDE 3

6 1. INTRODUCTION 1.1 The design of supports for traffic signs has changed significantly as a result of the introduction of European standards. This booklet attempts to clarify the current situation and to provide a reference for all those involved with traffic signing. Much of Appendix C requires some understanding of structural engineering, but the bulk of this publication is intended for use by traffic and highway engineers: the people who most often need to specify sign structures. In conjunction with suitable computer software, it will cover most situations. It is nevertheless recommended that the advice of a structural engineer should be sought in cases of any doubt and always for larger signs and those mounted above the carriageway. 1.2 For many years sign structure design in the UK was standardised, a single wind pressure being used throughout the country. However, the relevant standard, BS 873, was withdrawn at the end of 2005, being replaced initially by BS EN :2001 and now by BS EN :2007. One of the changes from BS 873 is the need to specify what wind pressure each sign needs to withstand. There is thus an additional step in the design process that this Guide helps to explain. 1.3 The European standard offers a wide range of classes for each aspect of a sign s performance, in order to address the requirements of all participating countries. To assist designers of traffic signs in the UK and to encourage consistency where it is appropriate, the UK version of the standard, BS EN :2007, has a National Annex appended to it. This contains recommended values and classes for the options, including for wind loading, as is therefore an invaluable reference. 1.4 The determination of the wind load and the design of the structure are two essentially separate tasks that do not necessarily need to be undertaken by the same person or organisation. It is recommended that when a wind load is recorded or communicated between organisations as part of a sign specification, this should be the basic wind pressure, w b. This is the load prior to the application of any safety factor or force coefficient, and is equivalent to and comparable with the values in Table 8 of BS EN :2007 and table NA.2 of its National Annex. As the nine WL classes in BS EN :2007 differ from those in the previous edition, the use of the basic wind pressure is recommended in preference to stating a WL class to avoid any confusion as to which version of the standard is intended, and to permit the range to be extended and intermediate values specified. 1.5 The UK National Annex to BS EN :2007 published on 30 April 2008 includes a table of suitable wind pressures, broken down by country or region and overall sign height. It does not address signs over 7 m total height, nor those at an altitude of more than 250 m above sea level. This advice is the result of work commissioned by the Highways Agency and significant discussion amongst industry experts. The intention was to produce guidance that was (a) reasonably simple to use, (b) complied with current standards on wind actions and (c) resulted in economic support sections that were comparable with those typically specified under previous standards. Inevitably this requires a much more detailed appraisal of the sign and its location than using a blanket wind load value. This booklet explains how to use this new method. 1.6 Those involved in designing the faces of traffic signs, particularly directional signs, should be aware that they frequently have a choice of layout and that the size and cost of the supporting structure will depend upon how they use this freedom. For example, a tall narrow sign requires stronger posts and foundations than a landscape format sign of the same area. Even where the available width is constrained, precluding a major rearrangement of the layout, it is often possible to reduce the sign face area by altering the positions of text and symbols to eliminate some of the blank space. In certain situations splitting directional information across two separate signs in advance of a junction is preferable to using a single large sign, for both driver perception and structural reasons. However, when attempting to reduce the size of a sign on no account should the size of lettering or the layout guidance 4 SIGN STRUCTURES GUIDE

7 in the Traffic Signs Manual and DfT working drawings be compromised, as this would degrade the readability of the sign and make it less suitable for its primary purpose of helping the road user. Sign structure design 1.7 Sizing the supports for a sign is a matter of balancing risk and economy. The most obvious risk is that a traffic sign structure might fail in strong winds, leading to the economic cost of replacing it and the safety and other disbenefits of the sign being absent in the meantime. There is also the danger that a failed or failing sign structure might cause damage or injury as it falls or as a result of it obstructing the highway. 1.8 However, the risk associated with the failure of a properly designed sign structure is negligible compared to the risk that a vehicle might leave the carriageway and collide with it. For this reason passively safe (or crash friendly) supports are increasingly specified for use on high-speed roads, or signs are placed behind safety fences. Highways Agency standard TD 19/06 requires safety barrier protection of roadside hazards mainly on roads with a 50 mph or higher speed limit, but the National Annex to BS EN 12767:2007 now recommends that passive safety should be a consideration on all roads. Signs designed to this standard must still meet the structural requirements of BS EN , although a greater deflection is permitted. There is always a safety benefit as well as an economic one in using the most slender support that is structurally adequate. 1.9 On all roads over-sized supports cost more, add to visual intrusion and are more likely to lead to death or injury than correctly designed ones. Over-design may also result in unnecessarily large foundations, leading to a longer construction period and thus greater disruption, coupled with the difficulty of locating such bases in urban roads containing extensive services. It is therefore well worth the trouble of designing sign structures using the method described in this publication, which aims to minimise support sizes whilst adhering to the relevant standards and sound engineering practice. Wind actions 1.10 The most significant force on a sign is exerted by the wind. Determining wind actions involves an element of probability and some knowledge of the physical characteristics and topography of the location concerned The relevant standard for wind actions is BS EN :2005, which is referenced by BS EN The UK National Annex to BS EN , expected to be published by summer 2008, gives a map of the UK showing the base reference wind speed for any location. To this value a number of corrections need to be applied to allow, for example, for the altitude, the height and ground clearance of the structure, its proximity to the coast or large inland lake and whether it is within a town or rural situation. Consideration also needs to be given to any local funnelling effect that might occur between tall buildings in a town environment, in a valley with a significant narrowing, or whether there is a sharp rise in ground level close to the relevant location. It is thus time consuming and generally uneconomic to derive an accurate wind load assessment individually for each sign location The National Annex to BS EN :2007 recommends suitable wind loads for the majority of signs in the UK. Whilst is it more conservative than performing a full analysis, it is simpler and quicker. The method is not suitable for all situations: it cannot be used at an altitude of greater than 250 m, or where there is significant orography or wind funnelling. For these situations the recommended action is to revert to BS EN This Guide consequently gives examples of both methods of calculation The National Annex suggests that authorities responsible for signs within a defined area may wish to derive a wind load that can be applied to all their signs, or to sub-divide their area into regions with similar wind characteristics. This will result in significant economy for areas located towards the south or east of their country or region or where the maximum altitude of any public highway is well below the 250 m assumed in the National Annex. It will also make subsequent design work easier for SIGN STRUCTURES GUIDE 5

8 authorities that have areas for which the simplified method cannot be used. The work involved in applying the methodology of BS EN a single time will be amply repaid for every sign that is subsequently designed to the wind load thus obtained In practice, most designers of sign structures will use appropriate computer software. They nevertheless need an overview of the methodology involved to ensure that they are using the software correctly, with suitable options and parameters selected, and to check that the results obtained are reasonable. A reference to software available free of charge is given in Appendix D (ref. 14). Other actions on signs 1.15 Other forces that may need to be taken into account when designing a sign structure are point loads and dynamic snow load. All versions of the UK National Annex recommend that signs should be able to withstand a force of 500 N applied at any point. This represents the load that might be exerted by, for example, a glancing blow from a vehicle mirror, a falling branch or malicious interference with the sign. This point load is the critical factor only for very small signs, but for signs mounted on a single support it causes torsional forces that need to be considered Dynamic snow load is relevant only to low-mounted signs in areas where snow ploughs or snow blowers are regularly used if there is a significant problem of sign damage caused by these operations. The BS EN National Annex recommends that class DSL1 (1500 N/m²) is relevant to snow blowing and class DSL2 (2500 N/m²) to snow ploughing at up to 40 mph. As snow clearance is likely to be regular only in areas of high wind load, and the dynamic snow load is not applied to any portion of the sign higher than 2.5 m from the ground, it will be unusual for the snow load to be the critical factor in sign structure design. It is therefore rare for snow loading calculations to be required. Comparison with previous methodologies 1.17 Prior to the introduction of BS EN 12899, the structures for most small and medium-sized traffic signs were designed using the DfT nomograms or a computer simulation of them. These charts with the reference WBM140 were first produced for the 1967 edition of Chapter 13 of the original Traffic Signs Manual and were last updated in They continued to be used, despite being based upon superseded British Standards, as there was no better guidance readily available The nomograms provided a choice of wind pressures and Chapter 13 recommended 1530 N/m² for signs in exposed positions and 960 N/m² for signs in urban areas and sheltered places. In the tables below 1500 N/m² has been used for consistency with BS 873. The columns headed England and Scottish mainland show the minimum suitable section following the UK National Annex method described in this Guide. Post diameters are in millimetres with the wall thickness bracketed. Where the overall height or centroid position of a sign requires a basic wind pressure different from that shown in the column heading, this value is indicated under the post size. Sign width height (mm) Mounting height (to lower edge) (m) Single CHS post (grade S275) Nomograms (WBM 140) 1500 N/m² England > 5 km from coast 1000 N/m² England < 5 km from coast 1100 N/m² Scottish mainland > 5 km from coast 1400 N/m² (4) 60 (4) 60 (4) 76 (3) (4) 114 (3) 114 (3) 114 (5) (5) 114 (5) 1200 N/m² 114 (5) 1300 N/m² 140 (4) 1600 N/m² 6 SIGN STRUCTURES GUIDE

9 Sign width height (mm) Mounting height (to lower edge) (m) 2 CHS posts (grade S275) Nomograms (WBM 140) 1500 N/m² (4) England > 5 km from coast 1200 N/m² 76 (4) 1000 N/m² England < 5 km from coast 1300 N/m² 76 (4) 1100 N/m² Scottish mainland > 5 km from coast 1600 N/m² 89 (4) 1400 N/m² (5) 89 (4) 114 (3) 114 (3) (5) 168 (5) 168 (5) 168 (6.3) (5.4) 194 (6.3) 194 (6.3) 219 (6.3) (6.3) 219 (6.3) 219 (6.3) 219 (8) Sign width height (mm) Mounting height (to lower edge) (m) 3 CHS posts (grade S275) Nomograms (WBM 140) 1500 N/m² England > 5 km from coast 1200 N/m² England < 5 km from coast 1300 N/m² Scottish mainland > 5 km from coast 1600 N/m² (5) 168 (6.3) 168 (6.3) 194 (6.3) (5.4) 194 (5) 194 (6.3) 219 (6.3) 1.19 The comparisons show that broadly similar sections are achieved in England, but that in Scotland a larger section than would have been used previously is generally indicated. The difference for signs on a single post is particularly marked. This is because the nomograms used a different deflection criterion for signs on a single support. For signs on two or more posts the nomograms were more stringent than BS 873 in imposing deflection limit of 12.5 mm/m BS EN and BS EN require the use of more conservative force coefficients and partial action (safety) factors than those adopted by the authors of the nomograms. Hence, for a typical small sign, a wind load of 1500 N/m² in the nomograms results in a broadly similar section size to a load of 1100 N/m² using the current method of calculation. SIGN STRUCTURES GUIDE 7

10 2. BACKGROUND INFORMATION ON WIND LOAD General 2.1 The National Annex to BS EN :2007 provides wind load values for the UK as an alternative to calculating loads using BS EN The wind pressure obtained using the table NA.2 (Appendix A) is conservative so, for large signs or where a number of signs are to be placed in a similar location, it is recommended that detailed calculations to BS EN be carried out. 2.2 The simplified method using the National Annex wind load values will take between ½ to ¾ hour depending on the complexity of the sign and location. This compares favourably with the alternative using BS EN to calculate the wind load, which can take several hours. The EN method is more complex and requires more detailed information about the location of the sign, which may not be readily available to the designer. It is possible to put much of the work into a spreadsheet to significantly reduce the design time, or to use suitable software that is available. It is also possible to calculate a wind loading that can be safely applied to all of a scheme area, county or maintenance area to avoid repeating this work for every sign. 2.3 In the examples the partial action factor recommended in the National Annex (class PAF1) is applied and an additional factor γ f3 of 1.0 included. γ f3 is traditionally used to allow for the possibility of inaccurate assessment of the effects of loading and unforeseen stress distribution in the structure. For a simple cantilever sign structure the possibility of inaccurate assessment of the effects of loading is limited, and, where the posts have been verified by static load testing unforeseen stress distribution in the structure, and variations in dimensional accuracy achieved in construction is also unlikely. Similarly for steel sections the reliability of the product is considered equivalent to static load tests so a factor of 1.0 is proposed. A designer may use this factor to allow for the possibility of future modifications to the sign or the addition of small signs to the post. 2.4 Designers will be able to achieve more efficient designs using the more rigorous method but the proposed National Annex simplified method will minimise the design effort for all but the more complex situations. 2.5 The example calculations are set out to match the format of limit state design codes. Load action load factors element capacity/material factor 2.6 There are two methods for calculating the exposure factor, c e (z). The Eurocode uses a terrain category and calculation of turbulence intensity and the National Annex to BS EN gives a look up table based on the distance from the shoreline. In both cases the wind load also depends on the height of the centroid of the sign from ground level. The look up wind loads presented in the National Annex Table NA2 are based on the more onerous results using both methods with the assumption of terrain category II. This terrain category is valid for all but the most exposed situations, but will be conservative for sheltered town sites. Aerodynamic Force Coefficient, c f 2.7 The National Annex to BS EN Table NA.2 (Appendix A) gives the force coefficient (also known as the shape factor) for different aspect ratios. This is based on a review of the available guidance and how it should be applied to signs. EN :2007 specifies the use of c f = 1.20 (clause 5.3.1). This conflicts with advice in BS EN clause and BS 6399 clause which both propose c f (or c p ) = 1.8 for signboards. The value of 1.8 is very conservative for signs, and it is valid to use an alternative method that considers them as structural elements with sharp edges. 2.8 Two different methods were reviewed for calculating c f : for elements with sharp edges. The first method uses BS EN Clause 7.7 and the other BS 6399 clause 2.7. The values in table NA2 in the 8 SIGN STRUCTURES GUIDE

11 National Annex to BS EN are based on the slightly more conservative results achieved using BS EN The BS EN Clause 7.7 method calculates c f = c f,o ψ λ with c f,o = 2 for all cases. The end effect is calculated from clause 7.13, which uses table 7.16 and figure 7.36 to obtain the factor. Section 1 is valid for signs as all will be less than 15 m long. We read the rule in this case as λ = 2 l/b, which is where this method is more conservative than the National Annex. Fig 7.36 gives a reduction factor based on the aspect ratio with a solidity ratio of 1 being valid for signboards The National Annex to BS EN will replace table 7.16 (ref. 1) with table NA.5 (ref. 3). This results in a change of formula for the slenderness ratio to: λ = l/b The limit values are as follows: Code l/b λ ψ λ c f EN NA to BS EN EN NA to BS EN Using this method no signs will have a c f close to It is important to note here that the reference height used in the design of elements is to the top of the element and not the centre of the area as it is for signs. This does not directly affect the calculations for c f but may be relevant when considering this method for use on signs. Design wind load to BS EN The examples in Appendix C show the detailed method for calculating wind load to BS EN and its UK National Annex. There are two methods for calculating c e (z). Both are based on the current (August 2007) draft National Annex to BS EN The first uses formula NA.3 using a terrain category of II, which does not use distance from the shoreline. The second method uses the shortened formula where orography is not significant and the factor for c e (z) is taken from table NA.7 for a known distance from the shoreline. The example calculations use the look up table NA.7, which is simpler to use The wind speeds are defined by the wind map in the National Annex (ref. 3). The height to the centroid is defined in BS EN fig 7.21 (ref. 1) and used in equation 4.8 in clause 4.5, which is then used in clause 5.3 equation 5.3. The wind force is therefore defined using the height to the centre of the sign. Note that equation 4.8 is replaced by equation NA.3 in clause NA 2.17 in the National Annex BS EN equation 5.3 clause 5.3 uses a structure factor c s c d which is calculated in equation 6.1 in Clause This factor is not used in the calculations as it is considered insignificant for traffic signs. The National Annex to BS EN clause NA 2.20 (ref. 3) gives guidance on the calculation of c s c d, which for most signs will be near The examples do not include the orography factor calculated to clause This is rarely significant but should be included at the calculation of q p (z). For areas with potential for wind funnelling expert advice must be sought. Wind Load values from the National Annex to BS EN 12899: The wind values given in Table NA.2 are based on calculations using both BS EN methods and taking the more conservative result. The height bands were reviewed to get the most efficient wind SIGN STRUCTURES GUIDE 9

12 value for each region. The limit on overall sign height of 7 m was chosen because HA standards require signs taller than this to have design and check certificate signed by a chartered engineer The wind values assume category II terrain roughness. Terrain categories are defined in BS EN :2005, clause 4.3.2, and category II and higher are the predominate terrain roughness parameters in the country. The wind load values would be excessively conservative if they all were based on the higher category I or 0. The values given cover the majority of situations with limitations of: maximum altitude; wind velocity grouping, etc. If a particular site falls in terrain category I or 0 then detailed design will be required. Exposure factors are related to the height, the smaller the height the greater the effect of the terrain roughness (terrain category). The relationship is not one that can be simplified into a general rule The reference heights for the wind values in the National Annex are from ground level to the top of the sign, rather than to the centroid. If the height to the centroid of the sign(s) is greater than ¾ H, then the limiting overall heights 4 m and 7 m in the table should be reduced to 3 m and 5.25 m respectively (unfortunately these heights are incorrectly stated as 3.25 m and 5 m in Note 7 to table NA.2 in the BS EN :2007 National Annex) The wind load values for a distance greater than 5 km from the shoreline in table NA.2 are valid except in the following areas: the small area approximately 10 miles to the south of Cape Wrath in northwest Scotland, Cornwall west of Falmouth, and Ynys Môn (Isle of Anglesey) in Wales. These areas should be treated as if they were entirely within 5 km of the shoreline The example calculations are based on the wind load values in the National Annex to BS EN :2007. The values for c f in the lookup table in the National Annex are based upon the EN procedure for thin structural elements, which is an improvement on the conservative value of 1.8 recommended for signboards. The method used to calculate c f is in accordance with BS EN which is slightly more conservative than the method in its National Annex. In the example calculations we have not shown interpolation from the table for c f though it would be acceptable The table NA.2 gives a maximum sign height of 7 m, which is rarely exceeded in practice. For the higher signs the designer should use the more rigorous method, as on trunk roads these will require certification as Category 0 structures in accordance with BD 2/ The Wind Load values in Table NA.2 are subject to the following caveats: The values are only valid up to an altitude of 250 m. The values are only relevant where there is no local funnelling effect, or significant orography as defined in clause and Annex A.3 of BS EN (cliffs / escarpments or sites with local funnelling effects). 10 SIGN STRUCTURES GUIDE

13 3. BACKGROUND INFORMATION ON SUPPORT DESIGN 3.1 The Ultimate Limit State (ULS) load effects are a combination of bending moment, shear and occasionally torsion. These are generally all at maximum at the top of the connection to the foundation. At the Serviceability Limit State (SLS) the structure has not permanently failed but is not performing adequately, in the case of a traffic sign because it is deflecting too much in the wind. For the less harmful SLS a lower wind pressure is used and the partial action (safety) factor is The design effects are calculated from simple analysis of loads on a cantilever including the load factors. 3.3 The designer should have access to the characteristic properties of the proposed supports. These can be developed from first principles by calculating the section area, second moment of area, etc., or from the manufacturer s published properties for individual products. The characteristic property is defined as that which 95% of the products will achieve (not the average). This is calculated by statistical analysis of test results when carried out. These values are reduced by a factor γ m (given in Table 7 of BS EN 12899:2007) to allow for the variability of the material. The selection of the support is based on the simple criterion that the capacity of the post must be greater than the load effect. 3.4 The example calculations take no account of wind pressure on the supports themselves, i.e. on the area below the sign plate that is exposed to wind. It has been shown that this has about 2% effect on the overall loads based on a height to the centroid of the sign, which is conservative. This is considered insignificant. If the designer wishes to include a value to allow for this it is suggested that use of γ f3 =1.05 would be adequate. 3.5 The calculations do not show an example where torsion may be significant. This may occur where the sign plate is positioned eccentrically to a single post or where the sign is part shielded by an obstruction that eliminates wind loading on part of it, or when a point load of 500 N (class PL3) on the extremity of the sign is considered. The use of an eccentricity of 0.25 the width of the sign (recommended in BS EN clause (2)) is considered excessive. It is suggested that the maximum torsion is achieved with the maximum wind loading on the area of sign plate one side of the post and that the associated bending be calculated using wind on only this area and not the whole sign. Where torsion is significant the sign support should be checked for a combined load effect. 3.6 The shear capacity check is given but in practice it is likely only to be significant for large signs at low mounting heights. 3.7 The deflection calculation is different to the previous method to BS 873, as it is checked at the top of the sign where maximum deflection occurs, not at the centroid. As the acceptable deflection is expressed in millimetres per metre height, this makes little difference in practice. SIGN STRUCTURES GUIDE 11

14 4. FOUNDATION DESIGN Design of spread foundations 4.1 The design of spread foundations is based on the calculation of stability against sliding and overturning of the base and maximum bearing pressure, while ignoring all passive resistance of the soil. The design uses unfactored loads and resistance and the factor of safety for overturning and sliding is calculated. These are considered safe if the factor of safety is greater than 1.5. The maximum bearing pressure is calculated on the assumption of poor soils, as experience has shown that little investigation is carried out prior to installing signs. It is good practice, but not essential, to aim for the resulting force to be within the middle third of the base. The equations for the alternative cases where the reaction is inside or outside the middle third will not be correct if the wrong one is chosen. For example, if q min is negative the reaction is outside the middle third. The most efficient base is achieved with minimum depth and length (parallel to the sign face), and increasing the width (parallel to the road) to achieve stability. It is recommended that bases for posts are reinforced to accommodate bending effects and pull-out of anchorages where used. This may be omitted for smaller signs and squarer bases where it can be shown that bending is not significant. 4.2 The allowable bearing pressure used in the calculations is the minimum value which permits design to proceed with limited ground investigations. It is possible to improve upon this for known founding material. Design of planted foundations to BD 94/ Planted foundations are based upon the method in the Highways Agency standard BD 94/07, which is also used for the design of lighting column bases. The design uses unfactored loads and resistance and calculates the factor of safety for overturning of the foundation. The stability of this type of foundation relies on the passive resistance of the soil surrounding the foundation which must be well compacted, and the required factor of safety is the lesser value of For this reason we do not recommend the use of planted foundations on a slope or near ditches or excavations without consulting a structural or geotechnical engineer. 12 SIGN STRUCTURES GUIDE

15 APPENDIX A: Table NA.2 of National Annex The IHIE regrets that for copyright reasons is not possible to include table NA.2 in this version downloaded from the Internet. To obtain a copy of the Guide containing table NA.2 please contact the IHIE at: [email protected] British Standards can be obtained in PDF or hard copy formats from the BSI online shop: or by contacting BSI Customer Services for hard copies only: tel: , [email protected] SIGN STRUCTURES GUIDE 13

16 The IHIE regrets that for copyright reasons is not possible to include table NA.2 in this version downloaded from the Internet. To obtain a copy of the Guide containing table NA.2 please contact the IHIE at: [email protected] 14 SIGN STRUCTURES GUIDE

17 The IHIE regrets that for copyright reasons is not possible to include table NA.2 in this version downloaded from the Internet. To obtain a copy of the Guide containing table NA.2 please contact the IHIE at: [email protected] SIGN STRUCTURES GUIDE 15

18 APPENDIX B: Flowchart for Determining Wind Load Start Is the site exposed / subject to local funnelling? Y Design to EN using 10 minute mean reference speeds N Height to Centroid ¾ H? Is Height 3 m? Is Height 5. 2 m? N N N Y Y Y Is Height 4 m? N Y Look up wind class using table NA 2 H = 4 m for location and distance from shoreline Is Height 7 m? N Y Look up wind class using Table NA 2 H = 7 m for location and distance from shoreline 1 : Calculate aspect ratio and look up c f ( Table NA 2 ) 2 : Look up partial action factors ( Tab le NA 2 ) 3: Calculate Wind Actions 4 : Look up partial material factors (Table NA 2) 5 : Calculate post resistance, using factored strength & properties. N Are wind load actions < post resistance? Y Finish 16 SIGN STRUCTURES GUIDE

19 APPENDIX C: EXAMPLES Introduction Two examples are examined: a small circular sign on a single support with a planted foundation, and a rectangular sign on two supports with a more conventional spread foundation. For each sign, the wind load is determined using both the simplified method of the National Annex to BS EN (section 1), and the more rigorous alternative method of BS EN (section 2). This enables the two approaches to be compared for complexity and results. These examples do not take into account the point load of 0.5 kn (class PL3 recommended in the National Annex). This is only likely to be critical for smaller signs on a single post, for which its effect should be checked. Whichever method is used to obtain it, the basic wind load is converted into a wind force for each of the states to be examined (section 3). These are used in section 4 for checking the sign supports. Finally, in section 5, the chosen foundation is assessed. EXAMPLE 1: a circular sign with a planted foundation Location Surrey, England, not within 5 km of the sea, nor a very exposed site on cliff / escarpment, nor in a site subject to wind funnelling. Mounting height above ground h m 2.0 m Width of sign face l 0.9 m Height of sign face b 0.9 m Total height = h m + b H 2.9 m Height to centroid of sign area = h m + b/2 z 2.45 m Depth of post buried above foundation h b 0 Foundation: support planted in plain concrete: minimum diameter of foundation in the ground. planting depth of foundation. D P SIGN STRUCTURES GUIDE 17

20 Example 1 Section 1: Basic Wind Load using BS EN National Annex 1. Look up wind load for site H = 2.9 m height, therefore Basic Wind Load, w b = 1.0 kn/m 2 (Used in section 3 below.) Table NA 2 Example 1 Section 2: Basic Wind Load using BS EN Look up fundamental value of basic wind velocity v b,0 = v b,map c alt = = m/s where v b,map = value of the fundamental basic wind velocity before the altitude correction is applied. (21.5 m/s selected from map) c alt = altitude factor c alt = A = = 1.25 A = Altitude of the site (m) above mean sea level (250 m in this example) Eqn NA1 Figure NA1 Eqn NA2a 2. Assess Terrain Orography Not very exposed site on cliff/escarpment or in a site subject to local wind funnelling. Therefore c o = 1.0 Fig NA2 3. Determine Design Life Requirement ( ln( 1 p) ) ( ln( 0.98) ) n 1 K ln cprob = = 0.96 Eqn K ln where p = design annual probability of exceedence p = 1/design life = 1/25 = 0.04 (for signs design life is 25 years) K = Shape parameter = 0.2 n = exponent = 0.5 NA Basic Wind Velocity v b = c dir c season v b,0 c prob v b = = 25.8 m/s Eqn 4.1 & Note 4 where c = directional factor = 1.0 dir c = season factor = 1.0 season NA 2.6 NA SIGN STRUCTURES GUIDE

21 5. Basic Velocity Pressure 1 2 qb = ρ vb = = kn/m 2 Eqn where ρ = air density = kg/m 3 NA Peak Velocity Pressure e,flat ( 2.45) c = 1.66 Fig NA7 q p ( z) ce,flat ( z) qb = = = 0.68 kn/m 2 NA 2.17 Modified formula where orography is not significant (c o = 1.0) for z = 2.45m and country terrain category, flat and 5km from the shore 7. Basic Wind Load w b = q p ( z) = 0.68 kn/m 2 (Used in section 3 below.) (This falls within BS EN :2007 class WL3, but use of these classes is no longer recommended.) Example 1 Section 3: Wind Force (for either method) 1. Determine force coefficient λ = effective slenderness ratio of sign or aspect ratio λ = l/b or b/l = 0.9/0.9 = 1.0 Therefore c f = 1.26 Table NA 2 2. Identify Partial Action factors for Loads γ fl ULS (Bending and Shear) γ fl = 1.35 SLS (Deflection and stability) γ fl = 1.0 Table NA 2 Additional factor γ f3, taken as 1.0, but could alternatively be 1.1 (see paragraph 2.3.) 3. Calculate wind design pressure w e,d on the sign w e,d = w b c f γ fl γ f3 where w b is the basic wind pressure from section 1 or 2 above. The section 1 value of 1.0 kn/m 2 will be used for the remainder of this example. SIGN STRUCTURES GUIDE 19

22 w e,d (ULS) = = 1.70 kn/m 2 w e,d (SLS) = = 1.26 kn/m 2 4. Calculate Total Design Wind Force F w,d = w e,d A ref (A ref = area of sign) Clause 5.3 BS EN F w,d (ULS) = 1.70 π = 1.08 kn F w,d (SLS) = 1.26 π = 0.80 kn 2 5. Wind Force Equivalent to 1-year Return Period 2 2 The wind velocity for calculating the temporary deflection is 75% of the reference wind velocity, as it is based upon a 1 year mean return period. Clause EN F w,d (1 year) = F w,d (SLS) = = 0.45 kn (If EN method is used, the calculated values are: F w,d (ULS) = 0.74 kn F w,d (SLS) = 0.55 kn F w,d (1 year) = 0.31 kn ) Use above forces in Sections 4 & 5 Example 1 Section 4: Support design (for either method) 1. Ultimate Design Load F w,d (ULS) = 1.08 kn Section 3 above 2. Ultimate Load Effects Ultimate design bending moment per post, M d M d = Wind force lever arm to foundation / number of posts = F w,d (ULS) (z+h b ) / n = 1.08 ( ) / 1 = 2.65 knm where n = number of posts Ultimate design shear per post, V d, = Wind force / number of posts V d = F w,d (ULS) / n = 1.08/1 = 1.08 kn 20 SIGN STRUCTURES GUIDE

23 3. Support Properties Circular Hollow Section, CHS (S275 steel) is anticipated Characteristic member capacities M c = 4.22 knm P v = 72.6 kn Steelwork design guide to BS :2000. Volume 1 Part C 4. Partial Material Factor for Sign Support γ m = 1.05 for steel sections (elongation > 15%) NA to EN Table NA.2 5. Ultimate Capacity Check Ultimate bending capacity = M c / γ m = 4.22 / 1.05 = 4.02 knm 4.02 knm > 2.65 knm OK (If EN method is used, the calculated bending effect is 1.81 knm) Ultimate shear capacity = P v / γ m = 72.6 / 1.05 = kn kn > 1.08 kn OK (If EN method is used, the calculated shear effect is 0.74 kn) 6. Temporary Deflection Calculation F w,d (1 year) = 0.45 kn Section 3 above Uniformly distributed load along sign face = F w,d (1 year) / b F w,d (1 year) / b = 0.45/0.9 = 0.5 kn/m where b = height of the sign face Maximum deflection at top of sign (bending), δ Clause EN δ = F w,d (1 year) 24EI n / b [ 3( H + h ) 4( h + h ) ( H + h ) + ( h + h ) ] b m b b m b Steel Designers Manual (Deflection table for cantilevers) where E = Elastic modulus of structural steel, taken as N/mm 2 I = Moment of inertia = 48.8 cm 4 (for CHS ) = mm 4 n = number of posts H, h b and h m as illustrated on diagram Steelwork design guide to BS :2000. Volume 1 Part B1 SIGN STRUCTURES GUIDE 21

24 δ = [ 3 ( ) 4 ( ) ( ) + ( ) ] δ = mm 4 1 Deflection per linear metre, δ = δ ( H + hb ) = ( ) = 9.72 mm/m Maximum temporary deflection taken as TDB4 = 25 mm/m 25 mm/m > 9.72 mm/m OK (If EN method is used, the calculated deflection is 6.61mm/m) NA to EN Table NA.2 EN Table 16 Conclusion: CHS (S275) is sufficient for the design. A smaller section might also be suitable. Notes: Design forces may also include the moment and shear force from the loads applied on the post(s). Torsion has not been calculated. This should be considered for signs that are fixed eccentrically, or where a significant area could be shielded from the wind or where buffeting from traffic can occur. Example 1 Section 5: Foundation Design to BD 94/07 1. Un-factored Design Load F w,d (SLS) = 0.80 kn Section 3 above 2. Ground Resistance Moment, M g M g G D P = 10 3 BD 94/07 Clause 11.5 where G is a factor dependent on the ground in which the support is planted (in kn/m 2 per m). Refer to BD 94/07 Table 3 for typical values of G. D is the minimum diameter of the support and its surrounding concrete in the ground (in m). P is the planting depth Try foundation design D = 0.35m, P = 0.8m 22 SIGN STRUCTURES GUIDE

25 Assume Poor ground condition, G taken as 230 kn/m 2 per m Table 3 M 3 g = = knm 3. Destabilising Moment, M DS The destabilising moment is calculated about a fulcrum point located at 1/ 2 of the planting depth below ground. Clause lever arm = [ z + hb + ( P)] 2 where z, h b and n are as defined before P is the planting depth M DS = F w,d lever arm to fulcrum point / number of posts 1 = F w,d [ z + hb + ( P)] 2 / n 1 = 0.80 ( ) / 1 = 2.41 knm 2 γ s;d M DS = = 3.01 knm where γ s;d is the model factor = 1.25 Clause Capacity Check M g = 4.12 knm > γ s;d * M DS = 3.01 knm OK Clause 11.6 Conclusion: A foundation of un-reinforced concrete, 0.35 m diameter with 0.8 m planting depth is satisfactory. Note: The design above does not apply to foundations on slopes, where stability of the ground needs to be taken into account. In such instances, specialist geotechnical advice should be sought. Clause 11.1 SIGN STRUCTURES GUIDE 23

26 EXAMPLE 2: a rectangular sign with a spread foundation Location Londonderry, Northern Ireland not within 5 km of the sea, nor very exposed site on cliff / escarpment, nor in a site subject to wind funnelling Mounting height above ground h m 1.5 m Width of sign face l 4.0 m Height of sign face b 2.5 m Total height = h m + b H 4.0 m Height to centroid of sign area = h m + b/2 z 2.75 m Depth of post buried above foundation h b 75 mm Spread foundation: length of foundation parallel to sign width of foundation perpendicular to sign thickness of foundation L W T Example 2 Section 1: Basic Wind Load using BS EN National Annex 1. Look up wind load for site H = 4.0m height, therefore Basic Wind Load Value, w b = 1.2 kn/m 2 (Used in section 3 below.) Table NA.2 24 SIGN STRUCTURES GUIDE

27 Example 2 Section 2: Basic Wind Load using BS EN Look up fundamental value of basic wind velocity v b,0 = v b,map c alt = = m/s where v b,map = value of the fundamental basic wind velocity before the altitude correction is applied. (26.25 m/s selected from map) c alt = altitude factor c alt = A = = A = Altitude of the site above mean sea level (217 m in this example) Eqn NA1 Figure NA1 Eqn NA2a 2. Assess Terrain Orography Not very exposed site on cliff/escarpment or in a site subject to local wind funnelling. Therefore c o = 1.0 Fig NA2 3. Determine Design Life Requirement ( ln( 1 p) ) ( ln( 0.98) ) n 1 K ln cprob = = 0.96 Eqn K ln where p = design annual probability of exceedence p = 1/design life = 1/25 = 0.04 (for signs design life is 25 years) K = Shape parameter = 0.2 n = exponent = 0.5 NA Basic Wind Velocity v b = c dir c season v b,0 c prob v b = = m/s where c = directional factor = 1.0 dir c = season factor = 1.0 season Eqn 4.1 & Note 4 NA 2.6 NA Basic Velocity Pressure 1 2 qb = ρ vb = = kn/m 2 2 Eqn 4.10 where ρ = air density = kg/m 3 NA Peak Velocity Pressure ( 2.75) c = 1.74 Fig NA7 e,flat SIGN STRUCTURES GUIDE 25

28 q p ( z) ce,flat ( z) qb = = = 1.00 kn/m 2 NA 2.17 Modified formula where orography is not significant (c o = 1.0) for z = 2.75m and country terrain category, flat and 5km from the shore. 7. Basic Wind Load w b = q p ( z) = 1.00 kn/m 2 (Used in section 3 below.) (This is equivalent to BS EN :2007 class WL5, but use of these classes is no longer recommended.) Example 2 Section 3: Wind Force (for either method) 1. Determine force coefficient λ = effective slenderness ratio of sign or aspect ratio λ = l/b = 4.0 / 2.5 = 1.6 Therefore c f = 1.30 Table NA 2 2. Identify Partial Action factors for Loads γ fl ULS (Bending and Shear) γ fl = 1.35 SLS (Deflection and stability) γ fl = 1.0 Table NA 2 Additional factor γ f3, taken as 1.0, but could alternatively be 1.1 (see paragraph 2.3). 3. Calculate wind design pressure w e,d on the sign w e,d = w b c f γ fl γ f3 where w b is the basic wind pressure from section 1 or 2 above. The section 1 value of 1.2 kn/m 2 will be used for the remainder of this example. w e,d (ULS) = = 2.11 kn/m 2 w e,d (SLS) = = 1.56 kn/m 2 4. Calculate Total Design Wind Force F w,d = w e,d A ref (A ref = area of sign) Clause 5.3 BS EN F w,d (ULS) = = 21.1 kn F w,d (SLS) = = 15.6 kn 26 SIGN STRUCTURES GUIDE

29 5. Wind Force Equivalent to 1-year Return Period The wind velocity for calculating the temporary deflection is 75% of the reference wind velocity, as it is based upon a 1 year mean return period. Clause EN F w,d (1 year) = F w,d (SLS) = = 8.78 kn (If EN method is used, the calculated values are: F w,d (ULS) = 17.6 kn F w,d (SLS) = 13.0 kn F w,d (1 year) = 7.31 kn ) Use above forces in Sections 4 & 5 Example 2 Section 4: Support design (for either method) 1. Ultimate Design Load F w,d (ULS) = 21.1 kn Section 3 above 2. Ultimate Load Effects Ultimate design bending moment per post, M d M d = Wind force lever arm to foundation / number of posts = F w,d (ULS) (z+h b ) / n = 21.1 ( ) / 2 = knm where n = number of posts Ultimate design shear per post, V d V d = Wind force / number of posts = F w,d (ULS) / n = 21.1 / 2 = kn 3 Support Properties Circular Hollow Section, CHS (S275) is anticipated Characteristic member capacities M c = 33.7 knm P v = 254 kn Steelwork design guide to BS :2000. Volume 1 Part C SIGN STRUCTURES GUIDE 27

30 4. Partial Material Factor for Sign Support γ m = 1.05 for steel sections (elongation > 15%) NA to EN Table NA2 5. Ultimate Capacity Check Ultimate bending capacity = M c / γ m = 33.7 / 1.05 = knm knm > knm OK (If EN method is used, the calculated bending effect is knm) Ultimate shear capacity = P v / γ m = 254 / 1.05 = kn kn > kn OK (If EN method is used, the calculated shear effect is 8.78 kn) 6. Temporary Deflection Calculations F w,d (1 year) = 8.78 kn Section 3 above Uniformly distributed load along sign face = F w,d (1 year) / b F w,d (1 year) / b = 8.78 / 2.5 = 3.51 kn/m where b = height of the sign face Maximum deflection at top of sign (bending), δ Clause EN δ = F w,d (1 year) 24EI n / b [ 3( H + h ) 4( h + h ) ( H + h ) + ( h + h ) ] b m b b m b Steel Designers Manual (Deflection table for cantilevers) where E = Elastic modulus of structural steel, taken as N/mm 2 I = Moment of inertia = 856 cm 4 (for CHS ) = mm 4 n = number of posts H, h b and h m as illustrated on diagram Steelwork design guide to BS :2000. Volume 1 Part B1 δ = [ 3 ( ) 4 ( ) ( ) + ( ) ] δ = mm Deflection per linear metre, δ = δ ( H + hb ) = ( ) = 7.87 mm/m 28 SIGN STRUCTURES GUIDE

31 Performance Class Maximum temporary deflection taken as TDB4 = 25 mm/m 25 mm/m > 7.87 mm/m OK NA to EN Table NA.2 EN Table 16 (If EN method is used, the calculated deflection is 6.56 mm/m) Conclusion: 2 no. CHS (S275) are sufficient for the design. (If EN method is used, a support size of CHS with M c = 27.8 knm, P v = 261 kn and I = 589 cm 4 is suitable.) Notes: Design forces may also include the moment and shear force from the loads applied on the post(s). Torsion has not been calculated. This should be considered for signs which are fixed eccentrically on the posts, where a significant area could be shielded from the wind or where buffeting from traffic can occur. Example 2 Section 5: Foundation Design to BS 8002 and BS Un-factored Design Load F w,d (SLS) =15.6 kn Section 3 above 2. Assumptions Presumed allowable ground bearing pressure = 80 kn/m 2 BS 8004 Table 1 Friction factor, µ taken as 0.5 (typical value for concrete against gravel, loose sand and firm clay) BS 8002 Clause Unit weight of reinforced concrete g con taken as 24 kn/m 3 3. Foundation dimensions Try foundation dimensions L = 3.0 m, W = 1.7 m, T = 1.2 m L, W and T as illustrated and defined above. Foundation self-weight, N = = kn SIGN STRUCTURES GUIDE 29

32 4. Overturning Check Overturning moment, M over = Design load lever arm to the foundation base M over = F w,d (z + h b + T) = 15.6 ( ) = knm Restoring moment, M rest = N W / 2 M rest = / 2 = knm Factor of safety = / = 1.99 > 1.5 OK 5. Bearing Pressure Check Eccentricity of resulting force, e = M over / N = / = 0.43 m W/6 = 1.7 / 6 = 0.28 m 0.43 > 0.28 e > W/6 reaction is outside the middle third Maximum bearing pressure, q max = 4N 3L ( W 2e) q max = ( ) = kn/m 2 q max < allowable bearing pressure, 80 kn/m 2 OK Note: if e < W/6, use these formulae: N 6e N 6e q max = (1 + ) q min = (1 ) q min must be 0 WL W WL W 6. Sliding Check Sliding force, F w,d = 15.6 kn Sliding resistance = N µ = = kn Factor of safety = / 15.6 = 4.7 > 1.5 OK Conclusion: Foundation with L = 3.0m, W = 1.7m and T = 1.2m is sufficient for the design. A spread foundation should be reinforced unless it is shown that the bending and shrinkage effects are negligible. In general foundations for smaller CHS posts (88.9 mm diameter or less) do not need reinforcement. Refer to Example 1 for planted bases, which are often an economical alternative. 30 SIGN STRUCTURES GUIDE

33 APPENDIX D: REFERENCES 1. BS EN : 2005, Eurocode 1: General Actions Wind Actions. 2. BS EN :2007 Fixed, vertical road traffic signs Part 1: fixed signs. 3. National Annex to BS EN , Eurocode 1: Actions on Structures. Draft dated August BS 6399:1997, Loading for Buildings, Part 2 Code of practice for Wind loads. 5. BS 8002:1994, Code of practice for earth retaining structures 6. BS 8004:1986, Code of practice for foundations 7. Steelwork design guide to BS :2000 (vol 1) 8. Steel Designers Manual, Steel Construction Institute, Blackwell Publishing, BS EN 12767:2007, Passive safety of support structures for road equipment. 10. National Annex to BS EN 12767:2007, Passive safety of support structures for road equipment. 11. TD 19/06 (vol 2, sect 2, part 8 of Design Manual for Roads and Bridges), Highways Agency. 12. National Annex to BS EN :2007 Fixed, Vertical Road Traffic Signs. 13. BD 94/07 (vol 2, sect 2, part 1 of Design Manual for Roads and Bridges), Highways Agency. 14. Free software for determining both the appropriate wind load and suitable steel support sections using the methods described in this booklet (SignLoad Designer) is available from Buchanan Computing: SIGN STRUCTURES GUIDE 31

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35 Disclaimer: The Institute of Highway Incorporated Engineers and the contributors which produced this Guide have endeavoured to ensure the accuracy of its contents. However, the guidance and recommendations given should always be reviewed by those using the guidance in the light of the facts of their particular case and specialist advice should be obtained as necessary. No liability for negligence or otherwise in relation to this Guide and its contents can be accepted by the Institute, the Contributors, its servants or agents.

36 Contributing organisations Highways Agency Buchanan Computing Mott MacDonald